A conventional missile or even missiles is not going to destroy a huge gravity dam like that. They are incredibly tough structures and missile warheads aren’t big. We’re talking concrete hundreds of feet thick.
So unless Taiwan has a method to deliver something the size of a bunker buster to the underwater base of the upstream side of the dam I don't think it's going to happen. And if they did manage to pull it off they'd presumably be condemned as war criminals more or less universally.